The invention relates to a method for producing an electrical contact element of the type in which at least one contact piece is joined to a carrier by welding.
In order to avoid welding during electrical switching operations of the contact elements, it is desirable that the contact pieces are produced from contact materials which do not weld easily. If such contact elements are used, difficulty can be experienced when joining the contact pieces to the carrier by brazing or welding. This is particularly true of composite materials comprising silver containing non-metallic inclusion, for example, AgCdO, AgSnO.sub.2, AgMgO and AgC, that are often used as contact materials. These contact materials contain, embedded in the silver matrix material, non-metallic particles of cadmium oxide, tin oxide or magnesium oxide, for example, and these inclusions may take the form of fibre-like strands lying at right-angles to the eventual contact surface of the contact piece. Although the non-metallic inclusions have the desired effect of offering considerable resistance to welding during the switching operation, their use leads to considerable difficulties in the joining of the contact materials when the existing welding or brazing methods are employed.
It is known, for the purpose of securing contact pieces of fusion-resisting material to a carrier, to provide the contact piece with a readily weldable or brazable layer on its rear face i.e. that face which faces the carrier. Intermediate layers of this kind can be produced by various known methods. In the case of internally oxidized material, it is possible to form the weldable or brazable layer by, for example, oxidation on the rear face of the silver layer or by prior cladding of the rear face. Another known possible method involves the production by powder metallurgy of a brazable or weldable intermediate layer by multi-layer pressing or by converting multi-layer ingots to the required size and shape. The production of such contact pieces having weldable or brazable rear face is very complicated and costly.
It is also known to connect contact materials, having poor welding properties, directly to a carrier by ultrasonic welding. The ultrasonic welding process offers advantages as regards the low thermal loading of the contact material, but in many cases does not result in satisfactory shear strength when the contact pieces and the carrier material are directly joined to each other. Certain favourable combinations of material, i.e. AgCdO and brass, cannot be joined together sufficiently firmly by means of the existing ultrasonic welding techniques.
One ultrasonic welding method includes a procedure for joining aluminum electric conductors wherein the contact zones are first covered with a contact foil by ultrasonic spot-welding of ultrasonic roller-welding. A soldered, screw-type or clamp-type connector is then provided at the connection zones thus prepared. Composite foils of copper and aluminum, or gold or silver foils are used as the contact foils. The connection of contact pieces to the carrier, previously clad by means of ultrasonic welding, is difficult and is costly to carry out on a mass-production basis.